1. Field of the Invention
The present invention relates to a propelling transmission control apparatus for a working vehicle such as an agricultural working vehicle, a transport vehicle, a lawn mower and the like utilizing a hydrostatic stepless transmission (HST) as a propelling change speed device.
2. Description of the Related Art
A hydrostatic stepless transmissions for use in a tractor exemplifying the above-noted working vehicles often employs a construction in which a swash plate angle of a hydraulic pump of the variable displacement type is varied by pedal operation to steplessly change an operating speed of a hydraulic motor of the fixed displacement type. The output speed of the motor is further changed into a plurality of speeds through a gear-type change speed device to drive wheels. In order to enable the operator to effect the pedal operation lightly and smoothly, a construction has been implemented in which the swash plate of the hydraulic pump is operable by a hydraulic servomechanism (see Japanese Patent Application “Kokai” No. 2002-283860, for example).
In general, a working vehicle using a hydrostatic stepless transmission as a propelling change speed device does not control speed in response to variations in propelling load. Instead, the operator is expected to effect a speed reduction control when he or she determines based on an engine sound that the engine is in an overload condition. Such a change speed operation as relying on the operator to determine engine load conditions would not only impose an excessive burden on the operator, but also lower operating efficiency.
In order to relieve the operator from the troublesome change speed operation to be effected in response to an engine load, research and development efforts are being conducted on a device for detecting load variations with a sensor from variations in the rotating speed of the engine, and automatically changing speed based on the load variations. However, such a device tends to result in an increased manufacturing cost since various sensors and complicated electric systems are required, and thus a disadvantage that the device may be installed only on high-end machines.